1. Field of the Invention
In a class of embodiments, the invention relates to a dual LCD panel display including two modulating LCD panels: an achromatic LCD panel and a color LCD panel. In a class of embodiments, the inventive dual LCD panel display includes an achromatic LCD panel, a color LCD panel, a backlight assembly, a cross BEF (“brightness enhancing film”) collimator between the backlight assembly and one of the color LCD panel and the achromatic LCD panel, and a polarization-preserving diffuser between the achromatic LCD panel and the color LCD panel.
2. Background of the Invention
Throughout this disclosure including in the claims, the expression performing an operation “on” signals or data (e.g., filtering, scaling, or transforming the signals or data) is used in a broad sense to denote performing the operation directly on the signals or data, or on processed versions of the signals or data (e.g., on versions of the signals that have undergone preliminary filtering prior to performance of the operation thereon).
Throughout this disclosure including in the claims, the noun “display” and the expression “display system” are used as synonyms. The expression “high dynamic range” display (HDR display) herein denotes a display having a dynamic range of greater than 800 to 1. Recent advances in technology have produced displays claiming contrast ratios of more than 1,000,000 to 1.
Throughout this disclosure including in the claims, the expression “dual LCD panel display” is used to denote a display system including two modulating LCD panels (an achromatic LCD panel and a color LCD panel), and a backlight system for illuminating the LCD panels. The backlight system can be a spatially variable backlight system (e.g., a spatially variable backlight panel comprising an array of individually controllable LEDs, or other spatially variable backlight panel) or a fixed backlight. The achromatic LCD panel and the color LCD panel are arranged so that one (a “first” one) of them is backlit by the backlight system and the other one of them is backlit by light transmitted through the first one of the LCD panels. A dual LCD panel display whose backlight system is a spatially variable backlight system is an example of a “dual modulation display” as defined herein.
Throughout this disclosure, the expression “dual modulation display” is used to denote a display system including a modulating front LCD panel system and a spatially variable backlight system (e.g., a spatially variable backlight panel comprising an array of individually controllable LEDs, or another spatially variable backlight panel) for backlighting the front LCD panel system. Examples of a modulating front LCD panel system of a dual modulation display include (but are not limited to) a single LCD panel comprising an array of LCD elements; and two LCD panels (an achromatic LCD panel and a color LCD panel) arranged so that one (a “first” one) of the LCD panels is backlit by the backlight system and the other one of the LCD panels is backlit by light transmitted through the first one of the LCD panels.
An optical element which is a film or layer having “uniaxial symmetry” herein denotes an optical element having a major surface which is at least substantially flat, and having a normal axis (perpendicular to the surface) and two orthogonal axes (sometimes denoted herein as “surface” axes or “X” and “Y” surface axes for convenience) that are perpendicular to the normal axis, and wherein the optical properties of the film or layer are at least substantially uniform with respect to a first one of the surface axes (the “X” surface axis) but not with respect to the other one of the surface axes (the “Y” surface axis). The major surface may be “at least substantially flat” although it has surface features (e.g., ridges) which are small relative to the overall size of the optical element and which make the surface non-flat on the (small) length scale of such features. An optical element which is a film or layer having “uniaxial symmetry” does not have radial symmetry in a plane orthogonal to its normal axis. In use in an optical system (e.g., a dual LCD panel display), an optical element which is a film or layer having “uniaxial symmetry” is typically oriented with its normal axis aligned at least substantially with the system's optical axis.
Herein, the expression “BEF” (or “brightness enhancing film”) denotes an optical element which is a film or layer having uniaxial symmetry and which is configured to selectively transmit and/or reflect incident light depending (in accordance with a first transfer function) on the light's angle of incidence in the plane defined by the optical element's normal axis and a first one of its surface axes (e.g., the “X” surface axis), but to have transmissivity and reflectivity that depend (in accordance with a different transfer function) on the angle of incidence in the orthogonal plane defined by the normal axis and the other one of the surface axes (e.g., the “Y” surface axis). A BEF may have features (e.g., ridges) which are small relative to the overall dimensions of its major surface, and which extend into and/or out from the surface parallel to the normal axis. Thus, a BEF would not have the same collimation functionality with respect to both of its orthogonal surface axes, although an optical element consisting of two identical BEFs with their major surfaces parallel to each other but oriented with their “X” surface axes offset with respect to each other by 90 degrees could have the same collimation functionality with respect to both of its orthogonal surface axes.
A typical BEF is a sheet (e.g., of polymer material) imprinted with a prismatic surface pattern, which resembles a saw tooth pattern extending inward and outward from the sheet's normal axis. The prisms cause the BEF to redirect (reflect) incident light (e.g., from the backlight of an LCD display) that is not incident at a high angle of incidence, and to transmit light that is incident at a high angle of incidence (e.g., light that is propagating almost perpendicular to the BEF's normal axis). Light reflected from the BEF may be re-reflect from other elements, so as to be incident again at the BEF this time with an incidence angle such that it is transmitted through the BEF (e.g., to increase the brightness of an image displayed by a backlit LCD display system including the BEF relative to the brightness that the image would have if the BEF were omitted from the display system).
An example of a BEF is a “DBEF” (dual BEF) film. A typical DBEF is a sheet (e.g., of polymer material) imprinted with a prismatic surface pattern extending inward and outward from the sheet's normal axis (as described in the previous paragraph) and also including an extra reflective layer for recycling (by reflection) light that is incident at the prisms but not at the correct angle for transmission.
Another example of a BEF is a “BEF-D” (BEF-diffuse) film, which is a multilayer film including a BEF layer (e.g., a DBEF layer, in which case the BEF-D film may be referred to as a “DBEF-D” film) and at least one diffusing layer. In use in a backlit LCD display system, a BEF-D may provide wider viewing angle than that which could be achieved if the BEF-D were replaced in the display system by a BEF film that is not a BEF-D film.
Several embodiments of dual LCD panel displays, other dual panel displays, and high dynamic range displays are described in U.S. patent application Ser. No. 12/780,749, filed on May 14, 2010, by Gopal Erinjippurath and John Gilbert. Several methods and systems for driving the achromatic LCD panel and color LCD panel of a dual LCD panel display are described in that application. U.S. application Ser. No. 12/780,749 notes that various optical elements may be placed at virtually any point in the light/image chain of such a dual LCD panel display, including any of diffusers, collimators, Brightness Enhancement Films (BEFs), and Dual Brightness Enhancement Films (DBEFs).
Contrast ratio is defined as the ratio of the brightest to darkest colors that a display is capable of producing. High contrast ratios are desirable for accurate image reproduction, but are often limited in traditional displays. One traditional display consists of a Liquid Crystal Display (LCD) panel and a backlight, typically a cold cathode fluorescent lamp (CCFL) disposed behind the LCD panel. The display contrast ratio is set by the LCD contrast ratio, which is typically under 1000:1. A dual LCD panel displays can provide a greater contrast ratio than can a traditional display or a dual modulation display that includes only a single LCD panel.
When dual modulation display or dual LCD panel display includes a spatially variable backlight system, the backlight drive values (e.g., LED drive values) should be chosen to achieve an optimal backlight, including by maximizing contrast, while minimizing visual artifacts (e.g., white clipping, black clipping, and halos) and temporal variations of these artifacts and maximizing energy efficiency. The ideal solution balances these criteria for a given application. Preferably, the backlight drive values control the backlight system to mitigate display artifacts such as bright pixel clipping, dark clipping and contouring, and output variation with motion and image deformation.
In a dual modulation display including a spatially variable LED backlight system, the contrast at the LCD front panel system is increased by multiplication by the contrast of the LED backlight. Usually, the backlight layer emits light corresponding to a low-resolution version of an image, and the LCD front panel system (which has a higher resolution) transmits light (by selectively blocking light from the backlight layer) to display a high-resolution version of the image. In effect, the high and low resolution “images” are multiplied optically.
In a dual modulation display including a spatially variable LED backlight system, nearby LCD pixels typically have similar backlighting. If an input image contains pixel values beyond the contrast range of an LCD panel, the backlight will not be optimal for all LCD pixels. Typically the choice of backlighting level for a local area of an LCD panel is not optimal for all LCD pixels in the area. For some LCD pixels the backlight might be too high, while for others the backlight might be too low. The backlighting should be set to best represent the input signal from a perceptual standpoint, i.e., the backlight level should be chosen to allow the best perceptual representation of the bright and dark pixels, which often cannot both be accurately represented.
If backlighting is too high, accurate low levels including black are compromised. Input image pixel values requiring LCD values near the minimum LCD transmittance are contoured (quantized), and pixels requiring LCD values below the minimum LCD transmittance are clipped to the lowest level. If the backlighting is too low, pixels above the backlight level are clipped to the maximum LCD level. These clipping and contouring artifacts may occur in traditional constant backlit LCD displays.
Examples of dual panel displays of the type described in U.S. patent application Ser. No. 12/780,749, each including an image-generating panel (e.g., a color LCD panel but alternatively another image-generating panel) and a contrast-enhancing panel (e.g., an achromatic LCD panel, but alternatively another contrast-enhancing panel), and a backlight, will be described with reference to FIGS. 1, 2, 2A, 3A, 3B, 4A, 4B, 4C, and 4D.